The fluid compartment that overlays the entire lung surface is maintained in a remarkably reduced state even during oxidant challenge. Lung surface extracellular antioxidants, such as ascorbate (AA) and glutathione (GSH) have been postulated as our first line of defense against oxidative stress. The current paradigm suggests that such species are maintained in reduced states by complex mechanisms that include surface breakdown (for GSSG), intracellular transport and reduction, reassembly (for GSH), and transport back to the surface. In this R21 application we hypothesize that the lung with its large surface area and thin aqueous lining layer is best suited for a Vectorial Trans-membrane Electron Transport System (VTETS), which would directly reduce extracellular species rather than the potentially slow and energy demanding sequence outlined above. However, direct trans-membrane shuttling could also have implications regarding extracellular redox cycling of inhaled toxicants. Compelling preliminary evidence suggests that lung surface VTETS exists, is robust, and proceeds for extended times within intact rat lungs. We will further characterize lung surface VTETS by using select one- and two-electron acceptors to elucidate VTETS mechanism(s), and a variety of inhibitors to identify its essential components. Further investigations will clarify the capacity of VTETS to drive redox-cycling of quinoid compounds, such as those present in combustion-generated particulate matter, within the lung surface extracellular compartment. Documentation of such activity may reveal a Janus face to an otherwise beneficial electron shuttle system. The concept of a lung surface VTETS has never been explored and thus its characterization will provide new insights regarding regulatory mechanisms that govern lung redox homeostasis. Results from these proposed exploratory, high risk, high impact studies have significant potential to found further investigations. The elucidation of novel cellular processes that contribute to toxicant-induced pathologic sequelae and further mechanistic insights regarding the pathogenesis and/or progression of chronic lung diseases provide opportunities for therapeutic interventions.